The subject matter disclosed herein relates to digital radiographic detectors (DRD), in particular, to a DRD and method of maintaining consistent operation thereof.
Stationary radiographic imaging equipment are employed in medical facilities (e.g., in a radiological department) to capture medical x-ray images on x-ray detectors. Mobile carts may also include an x-ray source used to capture x-ray images on a digital x-ray detector. Such medical x-ray images can be captured using various techniques such as computed radiography (CR) and digital radiography (DR) in radiographic detectors.
A related art DR imaging panel acquires image data from a scintillating medium using an array of individual sensors (pixels), arranged in a row-by-column matrix, in which each sensor provides a single pixel of image data. Each pixel generally includes a photosensor and a switching element that can be arranged in a co-planar or a vertically integrated manner, as is generally known in the art. In these imaging devices, hydrogenated amorphous silicon (a-Si:H) is commonly used to form the photodiode (photosensor) and the thin-film transistor (TFT) switch needed for each pixel. In one known imaging arrangement, a front plane has an array of photosensitive elements, and a backplane has an array of thin-film transistor (TFT) switches.
FIG. 1 is an exemplary diagram that shows a perspective view of an area detector behind a patient including rows and columns of detector cells in position to receive x-rays passing through the patient during a radiographic imaging procedure. As shown in FIG. 1, an x-ray system 10 in combination with a detector array 12 may include an x-ray tube 14 that is collimated to provide a directed x-ray beam 16 passing through an area 18 of a patient 20. The beam 16 is attenuated along its many rays by the internal structure of the patient 20 and is received by the detector array 12 that extends in two dimensions over a prescribed area (e.g., a plane) preferably perpendicular to a central ray of the x-ray beam 16.
The detector array 12 can be divided into a plurality of individual cells 22 that can be arranged rectilinearly in columns and rows. As will be understood to those of ordinary skill in the art, the orientation of the columns and rows is arbitrary, however, for clarity of description it will be assumed that the rows extend horizontally and the columns extend vertically.
In exemplary operations, the rows of cells 22 can be scanned one or more at a time by scanning circuit 28 so that exposure data from each cell 22 (i.e., an amount of electric charge) may be read by read-out circuit 30. Each cell 22 can independently measure an intensity of radiation received at its surface and thus the exposure data read-out from each cell 22 provides one pixel of information used to generate an image 24 as displayed on a monitor 26, normally viewed by the user. A bias circuit 32 can control a bias voltage applied to the cells 22.
Each of the bias circuit 32, the scanning circuit 28, and the read-out circuit 30, can communicate with an acquisition control and image processing circuit 34 that can coordinate operations of the circuits 28, 30 and 32, for example, by use of a processor included therein. The acquisition control and image processing circuit 34, can also control exemplary examination procedures, the x-ray tube 14 by turning it on and off, as well as controlling the tube current, and thus the fluence of x-rays in x-ray beam 16, and/or the tube voltage, and thus the energy of the x-rays in x-ray beam 16.
The acquisition control and image processing circuit 34 can provide image data to the monitor 26, based on the exposure data read out from each cell 22. Alternatively, acquisition control and image processing circuit 34 can manipulate the image data, store raw or processed image data (e.g., at a local or remotely located electronic memory) or export the image data.
Exemplary cells 22 may include a photo-activated image sensing element and a switching element for reading a signal from the image-sensing element. Image sensing can be performed by direct detection, in which case the image-sensing element directly absorbs the x-rays and converts them into charge carriers. However, in most commercial digital radiography panels indirect detection is used, in which an intermediate scintillator element converts received x-rays to visible-light photons that can then be sensed by a light-sensitive image-sensing element.
Examples of image sensing elements used in image sensing arrays 12 include various types of photoelectric conversion devices (e.g., photosensors) such as photodiodes (P-N or PIN type), photo-capacitors (MIS), photo-transistors or photoconductors. Examples of switching elements used for signal read-out include MOS transistors, bipolar transistors and p-n junction components.
DR detectors with amorphous or poly-crystalline photosensors, such as a-Si:H photosensors, require a power-up transition time from a zero-power state to a stable state ready for exposure. The time for the transition can be limited by the time required for the trap states in such photosensors to transition from a zero-bias state to a state capable of low-noise and stable operation. After some time in the zero-bias state, the traps in such photosensors can reach an equilibrium state corresponding to zero-bias. Upon power-up, such photosensors can transition to a reverse-bias state. Trap occupancy in the reverse bias state is considerably lower than in the zero bias state. In the transition from the zero bias state to the reverse bias state, the traps in such photosensors emit electrons and holes to the conduction band and valance band, respectively. The emission time constant for electrons and holes to be respectively emitted from trap states to the conduction and valance band depends on the energy difference between the trap energy and the respective band edge.
FIG. 2 is a schematic diagram 240 of a portion of a two-dimensional array 12 for the DR detector 40. The array of photosensor cells 212, whose operation may be consistent with the photosensor array 12 described above, may include a number of a-Si:H n-i-p photodiodes 270 and thin film transistors (TFTs) 271 formed as field effect transistors (FETs) each having gate (G), source (S), and drain (D) terminals. In embodiments of DR detector 40 disclosed herein, the two-dimensional array of photosensor cells 12 may be formed in a device layer that abuts adjacent layers of the DR detector structure. A plurality of gate driver circuits 228 may be electrically connected to a plurality of gate lines 283 which control a voltage applied to the gates of TFTs 271, a plurality of readout circuits 230 may be electrically connected to data lines 284, and a plurality of bias lines 285 may be electrically connected to a bias line bus or a variable bias reference voltage line 232 which controls a voltage applied to the photodiodes 270. Charge amplifiers 286 may be electrically connected to the data lines 284 to receive signals therefrom. Outputs from the charge amplifiers 286 may be electrically connected to a multiplexer 287, such as an analog multiplexer, then to an analog-to-digital converter (ΔDC) 288, or they may be directly connected to the ADC, to stream out the digital radiographic image data at desired rates. In one embodiment, the schematic diagram of FIG. 2 may represent a portion of a DR detector 40 such as an a-Si:H based indirect flat panel imager.
Incident x-rays, or x-ray photons, 16 are converted to optical photons, or light rays, by a scintillator, which light rays are subsequently converted to electron-hole pairs, or charges, upon impacting the a-Si:H n-i-p photodiodes 270. In one embodiment, an exemplary detector cell 222, which may be equivalently referred to herein as a pixel, may include a photodiode 270 having its anode electrically connected to a bias line 285 and its cathode electrically connected to the drain (D) of TFT 271. The bias reference voltage line 232 may control a bias voltage of the photodiodes 270 at each of the detector cells 222. The charge capacity of each of the photodiodes 270 is a function of its bias voltage and its capacitance. In general, a reverse bias voltage, e.g. a negative voltage, may be applied to the bias lines 285 to create an electric field (and hence a depletion region) across the p-n junction of each of the photodiodes 270 to enhance its collection efficiency for the charges generated by incident light rays. The image signal represented by the array of photosensor cells 212 may be integrated by the photodiodes while their associated TFTs 271 are held in a non-conducting (off) state, for example, by maintaining the gate lines 283 at a negative voltage via the gate driver circuits 228. The photosensor cell array 212 may be read out by sequentially switching rows of the TFTs 271 to a conducting (on) state by means of the gate driver circuits 228. When a row of the pixels 22 is switched to a conducting state, for example by applying a positive voltage to the corresponding gate line 283, collected charge from the photodiode in those pixels may be transferred along data lines 284 and integrated by the external charge amplifier circuits 286. The row may then be switched back to a non-conducting state, and the process is repeated for each row until the entire array of photosensor cells 212 has been read out. The integrated signal outputs are transferred from the external charge amplifiers 286 to an analog-to-digital converter (ΔDC) 288 using a parallel-to-serial converter, such as multiplexer 287, which together comprise read-out circuit 230.
This digital image information may be subsequently processed by image processing system 34 to yield a digital image which may then be digitally stored and immediately displayed on monitor 26, or it may be displayed at a later time by accessing the digital electronic memory containing the stored image. The flat panel DR detector 40 having an imaging array as described with reference to FIG. 2 is capable of both single-shot (e.g., static, radiographic) and continuous (e.g., fluoroscopic) image acquisition.
A TFT switch's threshold voltage (VT) is the voltage level which defines the transition between the “ON” and “OFF” states of the TFT switch. A gate voltage set to be less than the VT level represents the “OFF” configuration of the TFT. A gate voltage set to be greater than the VT level represents the “ON” configuration of the TFT, as illustrated in FIG. 3.
The discussion above is merely provided for general background information and is not intended to be used as an aid in determining the scope of the claimed subject matter.